1. Field of the Invention
The present invention relates to a thin-film transistor and its manufacturing method as well as to circuits and apparatuses constituted by using thin-film transistors.
2. Description of the Related Art
The thin-film transistor (hereinafter abbreviated as xe2x80x9cTFTxe2x80x9d) using a thin-film semiconductor is known. This is formed on a substrate by using a thin-film semiconductor, particularly a silicon semiconductor film.
The TFT is used in various kinds of integrated circuits, particularly the active matrix liquid crystal display device. In the active matrix liquid crystal display device, TFTs as switching elements are provided for respective pixel electrodes that are arranged in matrix form. There is known a version (called xe2x80x9cperipheral driver circuits integrated typexe2x80x9d) in which not only a matrix circuit but also peripheral driver circuits are constituted by using TFTs.
Examples of other uses of the TFT are various kinds of integrated circuits and multilayered integrated circuits (three-dimensional ICs).
As for a silicon film used in the TFT, it is simple and convenient to use an amorphous silicon film formed by a vapor-phase method such as plasma CVD. It can be said that this method has already been established almost completely.
However, the electrical characteristics of the TFT using an amorphous silicon film are far lower than those of the TFT using a single crystal semiconductor for common semiconductor integrated circuits. Therefore, at present, this type of TFT can be used for only limited purposes such as a switching element of an active matrix circuit.
The characteristics of a TFT using an amorphous silicon film can be improved by converting the amorphous silicon film into a crystalline silicon film. Silicon films having crystallinity other than a single crystal silicon film are called a polysilicon film, a microcrystal silicon film, etc.
Such a silicon film having crystallinity can be obtained by forming an amorphous silicon film and crystallizing it by heating (thermal annealing). This method is called a solid-phase growth method because conversion from an amorphous state to a crystal state is effected while the solid phase is maintained.
However, the silicon solid-phase growth has problems that the heating temperature and time need to be set at more than 600xc2x0 C. and more than 10 hours, respectively, and that it is difficult to use an inexpensive glass substrate.
For example, the Corning 7059 glass, which is commonly used in the active matrix liquid crystal display device, has a glass strain point of 593xc2x0 C. Therefore, in view of increasing the substrate area, it is problematic to subject this glass to thermal annealing of more than 600xc2x0 C. for a long time.
Further, the fact that the heating time for crystallization is more than 10 hours is problematic in terms of productivity.
To solve the above problems, the inventors have developed a technique in which a certain kind of metal element such as nickel or palladium is deposited, by a very small amount, on the surface of an amorphous silicon film and then heating is performed. According to this technique, crystallization can be completed by performing heating at 550xc2x0 C. for about 4 hours (refer to Japanese Unexamined Patent Publication No. 6-244103 (JP-A-6-244103).
Naturally a silicon film even higher in crystallinity can be obtained by annealing of 600xc2x0 C. and 4 hours.
This technique can produce a large-area crystalline silicon film on an inexpensive glass substrate with high productivity.
To introduce a very small amount of metal element (for accelerating crystallization), various methods are available such as depositing a coating of a metal element or its compound by sputtering (refer to JP-A-6-244104), forming a coating of a metal element or its compound by such a means as spin coating (JP-A-7-130652), and forming a coating by decomposing a gas containing a metal element by thermal decomposition, plasma decomposition, or the like (JP-A-7-335548).
It is also possible to introduce a metal element selectively, i.e., into a particular portion, and then cause crystal growth to proceed from the portion where the metal element is introduced to the periphery (called xe2x80x9clateral growth methodxe2x80x9d). Having directivity in crystal structure, a crystalline silicon film produced by this method exhibits much superior characteristics when used properly in connection with the directivity.
Although the above-described methods of forming a crystalline silicon film by using a certain kind of metal element (for instance, nickel) are much superior, it is known that they cause the following problems when a TFT is formed by using such a crystalline silicon film.
First, the structure of a common TFT will be described. FIG. 1 shows the structure of a typical n-channel TFT.
In the structure of FIG. 1, an active layer including a source region 102, a low-concentration impurity region 103, a channel region 104, a low-concentration impurity region (LDD region) 105, and a drain region 106 is provided on a glass substrate 101. Reference numerals 111, 112, and 114 denote an interlayer insulating film, a source electrode, and a drain electrode, respectively.
The TFT structure as shown in FIG. 1 is generally employed in which the low-concentration impurity region 105 that is less conductive than the drain region 106 is provided on the drain side, particularly in n-channel TFTs. The region 105, which is called an LDD (lightly doped drain) region, such roles as reducing a strong electric field applied between the channel region and the drain region, reducing the leak current, and suppressing the hot carrier effect.
The following problems arise when a TFT having the above structure is formed by using a crystalline silicon film that has been crystallized by utilizing a particular metal element of the above kind.
A TFT formed by using a crystalline silicon film that has been obtained by utilizing a metal element exhibits much superior characteristics as a whole; for instance, it is even superior to a TFT formed by using a crystalline silicon film that has been obtained by irradiation with laser light.
However, the case of utilizing a metal element is associated with a problem that the characteristics vary to a large extent when a number of TFTs are produced. Further, TFTs having marked deteriorations in characteristics are found though the rate of occurrence is low. These deteriorations in characteristics increase variations in device characteristics as a group of TFTs.
The variations in device characteristics are a serious problem in making an integrated circuit. In general, in making an integrated circuit, it is important that the characteristics be uniform among devices used as well as the characteristics of each device be superior.
An object of the invention is to provide a technique for obtaining a TFT having small variations in device characteristics in forming it by using a crystalline silicon film that has been obtained by utilizing a metal element.
According to knowledge of the inventors, the variations in device characteristics are caused by the metal element that was used in the crystallization, which means that the problem of the variations in device characteristics can be eliminated by removing the metal element selectively from a crystalline silicon film obtained.
Studies of the inventors have revealed that where the nickel element is used, it can be removed (or its influences can be eliminated) by performing, on a crystalline silicon film obtained, a heat treatment at more than about 900xc2x0 C. in an oxygen atmosphere containing chlorine at several percent.
By utilizing this technique, a TFT having superior characteristics can be obtained with vary small variations in characteristics. A patent application has already been filed for this technique (Japanese Patent Application No. 8-335152).
However, this technique still has a problem that an inexpensive glass substrate cannot be used because heating at more than 900xc2x0 C. is needed to remove the nickel element.
To enable use of inexpensive glass substrates (for instance, the Corning 7059 glass substrate and the Corning 1737 glass substrate), it is desirable that the process temperature be lower than 600xc2x0 C.
Experiments of the inventors have revealed that the following method is effective in removing the nickel element by a heat treatment of as low a temperature as about 600xc2x0 C. (which is a low temperature when compared with 900xc2x0 C.):
(1) First, while part of a crystalline silicon film that has been obtained by the action of a metal element is left, phosphorus ions are accelerated and implanted into the other portion.
(2) Then, a heat treatment of about 600xc2x0 C. is performed.
As a result, the metal element is moved to the phosphorus-ions-implanted region as if it were sucked out.
However, the removal of the metal element by the above method utilizes a phenomenon that the metal element is moved parallel with the film surface rather than it is moved perpendicularly to and removed through the film surface. Therefore, the metal element is gradually moved from a peripheral portion of a subject pattern to the region where phosphorus ions are implanted, wherein said subject pattern is a region where the phosphorus ions are not implanted into.
Therefore, this method is not suitable for removal of a metal element from the entire subject pattern when the subject pattern has a large area.
On the other hand, concentrated studies of the inventors on the influences of metal elements on the TFT characteristics have led to knowledge that the variations and deteriorations in device characteristics of a TFT that has been formed by using a crystalline silicon film crystallized by utilizing a metal element are greatly influenced by the metal element remaining in a region where a strong electric field is to be applied.
In the TFT shown in FIG. 1, the strongest electric field is applied to the drain-side low-concentration impurity region 105. Therefore, the variations and deteriorations in device characteristics of the TFT can be reduced by decreasing the concentration of a metal element in the region 105.
In the invention, the above-mentioned technique of accelerating and implanting phosphorus ions is used to reduce the concentration of a metal element in the drain-side low-concentration impurity region 105.
According to one aspect of the invention, there is provided a semiconductor device (a specific structure is shown in FIGS. 4A and 4C) having an active layer formed by a crystalline silicon film that has been crystallized by utilizing a metal element for accelerating crystallization of silicon, wherein a channel region 238, a drain region 236, and a low-concentration impurity region (in other words; a lightly doped drain region) 237 are formed in the active layer; the low-concentration impurity region 237 is formed between the channel region 238 and the drain region 236, and is doped with an impurity for imparting conductivity at a lower concentration than the drain region 236; and a concentration of the metal element in the low-concentration impurity region 237 is ⅕ (one fifth) or less of that in the drain region 236.
Where the semiconductor device is a p-channel TFT, the drain region has p-type conductivity and the low-concentration impurity region has lower p-type conductivity than the drain region, and wherein the drain region is doped with phosphorus. In this case, although the drain region has p-type conductivity, it is also doped with phosphorus for gettering the metal element.
It is preferred that the metal element for accelerating the crystallization is nickel. This is because nickel is superior in both effect and reproducibility.
The metal element may be one or a plurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.
According to another aspect of the invention, there is provided a manufacturing method of a semiconductor device, comprising the steps of forming a crystalline silicon film by utilizing a metal element for accelerating crystallization of silicon (see FIGS. 2A and 2B); doping, with phosphorus, at least a first region (221-224) to become a drain region of a thin-film transistor (see FIG. 3C); performing a heat treatment, to thereby cause the metal element existing in a second region (227-230) adjacent to the first region to be gettered in the first region (see FIG. 3D); and forming a low-concentration impurity region (a lightly doped drain region) in the second region by doping the second region with an impurity for imparting conductivity at a lower concentration than the first region (see FIG. 4A and/or FIG. 4B).
This manufacturing process can attain the object of the invention by making the concentration of the metal element in the low-concentration impurity region ⅕ (one fifth) or less of that in the drain region.
Plasma doping and ion implantation in which ions of a dopant element are accelerated and implanted are typically used as a doping method.
Other usable doping methods include a method in which a thin film of a dopant element is formed on the surface of a region to be doped, a method in which a desired region is doped by exposing a sample to a plasma atmosphere containing a dopant element, a method in which a sample is illuminated with laser light in an atmosphere containing a dopant element, and a method in which a dopant element is caused to diffuse into a particular region of a film by applying a solution containing the dopant element to the film.